Cellular plastics have been available for many years. One of the first of such materials was cellular rubber dating to the 1910-1920 period. Subsequently, cellular compositions were made from latex, phenol-formaldehyde resins, urea-formaldehyde resins, PVC, polyurethane, cellulose acetate, polystyrene, polyethylene, epoxides, ABS resins, silicones and very recently polyphosphazenes. Polyphosphazene foams have very desirable properties in that they are highly fire resistant and when subjected to direct flame do not produce toxic smoke which is encountered with many other common foamed materials, notably, polyurethanes.
Polyphosphazenes are polymers containing a plurality of ##STR1## groups wherein substituents are bonded to phosphorus. The polyphosphazenes which are the concern of this invention are high molecular weight linear polyphosphazenes containing 50 or more of the above units and having molecular weights from about 10,000 up to about 5,000,000 or higher. They are substantially linear and have little, if any, cross-linking. In general, they are soluble in benzene, toluene, cyclohexane and tetrahydrofuran and are relatively insoluble in linear aliphatic hydrocarbons such as hexane or heptane. Groups substituted on phosphorus include phenoxy, alkylphenoxy, alkoxyphenoxy, aminoalkylphenoxy, alkylaminoalkylphenoxy, dialkylaminoalkylphenoxy, halophenoxy (e.g., para-chlorophenoxy, meta-bromophenoxy, trifluorophenoxy and the like), haloalkylphenoxy (e.g., trifluoromethylphenoxy), alkoxy, haloalkoxy (e.g., trifluoroethoxy), alkenylphenoxy (e.g., ortho-allylphenoxy and the like).
Methods of making cellular polyphosphazenes are known. Various procedures are described in U.S. Pat. Nos. 4,026,838; 4,055,520; 4,055,523; 4,107,108; 4,189,413; 4,536,520 and others. In general, the foams are made by mixing the polyphosphazene gum, a blowing agent and a peroxide or sulfur-type curing agent and heating the blended components to activate the blowing agent and cure the resultant foam.
Because these foams are widely recognized to possess the excellent flammability properties required for demanding applications such as pipe insulation and cushions, they have become items of considerable commercial significance. However, due to their flexible elastomeric nature they are potentially unsuitable for many foam-specific applications which require good thermal insulating materials having excellent flammability properties, but depend on a rigid foam to achieve the desired end-use performance as in the case of rigid pipe insulation, for example, where a flexible insulation would not take the compressive loads imposed by the application design or in the case of composite core material for use in aircraft, marine and aerospace applications. Also, it would be highly desirable if such a rigid polyphosphazene foam could be produced in a wide variety of shapes including rigid slabstock foam sheets and pipe insulation having varying degrees of curvature or complex contours to accommodate specific end uses. In response to this need, there is now provided a low density rigid polyphosphazene foam having excellent flammability and compression resistant properties which can be produced in a wide variety of shapes and designs.